This invention relates to a hole sensing device which is particularly useful in a spike driving machine for driving spikes through holes in tie plates to secure rails to ties.
Such a hole sensing device has been proposed in which a finger is slid along the upper surface of a tie plate, the finger operating an electric switch when the finger engages a hole in the tie plate thus providing an electric signal indicative of a sensed hole. This signal may then be sent to a reference mechanism which allows the sweep to continue until a spike setter and spike drive head following the hole sensing device become aligned with the hole at which point the sweep is discontinued.
A problem with the known device is that it is set to operate on a rail tie which is a particular height so that the switch is open while the finger engages the rail tie. However, sometimes a rail tie may be lower than the particular height for which the hole sensing device is set probably because of a tie which is inadvertently lower than the others. When the hole sensing device arrives at the low tie plate the finger drops further than normal and senses a hole anywhere on the tie plate. This ultimately causes a spike to be driven into the tie plate at a location where there is no hole causing damage to the tie plate, spike and the spike driver and setter.